The present invention relates to a multiplex transmission system using a CSMA/CD (Carrier Sense Multiple Access/Collision Detection) transmission system.
There has been proposed a multiplex transmission system using a CSMA/CD transmission system, which has a plurality of nodes mutully coupled through a transmission path, transmits data from any one of the nodes for each frame having a destination address, and returns a reception acknowledge signal to the transmitting node from a destination node specified by the destination address upon proper reception of the data by the destination node.
Conventionally, various types of such multiplex transmission systems have been proposed. The article "A Proposal for a Vehicle Network Protocol Standard" presented in the SAE International Congress and Exposition (Feb. 1986), discloses a physical message type multiplex transmission system as the first prior art system, in which in order to transmit data from one of a plurality of nodes coupled to a transmission path to another node, a physical address of the destination node is written in a destination designation area of a frame of data, this data frame is transmitted on the transmission path and the destination node specified by the address returns a reception acknowledge signal to the transmitting node upon reception of the frame.
For instance, in the case where this prior art system is applied to controlling the operation of head lamps, small lamps, turn-signal lamps, a horn and the like of a vehicle, it may be desirable that the same vehicle driving information is transferred from one of a plurality of nodes, such as a front multiplex node coupled to the head lamps, small lamps, turn-signal lamps, horns, etc. provided on the front side of the vehicle, a combination switch node provided with a number of switches to turn on or off these components, a meter node for indicating the ON/OFF states of these components, a rear multiplex node coupled to the small lamps, turn-signal lamps, etc. provided on the rear side of the vehicle, to some or all of the remaining nodes. In such a case, sequentially transmitting the same vehicle driving information to the individual nodes provides a significantly low transmission efficiency.
The shortcoming is prevented by providing a so-called simultaneous multi-destination transmission function to the transmission system in which a so-called global address is written in the aforementioned destination designation area of the frame to specify all of the destination nodes and the same data is simultaneously transmitted to all the destination nodes. In this case, if, upon reception of the frame from the transmitting node, all of the destination nodes send their acknowledge signals on the transmission path, a collision would occur. To prevent this collision, therefore, each node is provided with a collision detection function and a transmission inhibition function so that upon proper reception of the frame from the transmitting node, only the reception acknowledge signal of the destination nodes which has the highest priority, can survive to be returned to the transmitting node.
Although this method can solve the above problem concerning with the transmission efficiency, it provides no means to confirm that all the nodes requiring the data have properly received, thus resulting in a low reliability. With those nodes which do not need the data, it is also necessary to discriminate whether or not they should receive the transmitted signal, thus increasing the load of a controller which controls the circuit for executing the multiplex transmission of the individual nodes.
The second prior art system (functional message type) is also disclosed in the aforementioned article "A Proposal for a Vehicle Network Protocol Standard", in which a so-called function address is written in the destination designation area of a frame, a single function address being associated with a plurality of predetermined nodes, each reception node compares the function address of the frame sent on the transmission path from the transmitting node with a reception function address table stored in the reception node to discriminate whether or not it should receive the frame, and, if affirmative, it returns its own address as a reception acknowledge signal to the transmitting node. In this case, if a plurality of reception nodes simultaneously return their reception acknowledge signals, the aforementioned collision would occur. To avoid the collision, therefore, this system is also provided with the aforementioned collision detection and transmission inhibition functions as well as a reception acknowledge signal retransmission function, so that the reception nodes sequentially return their reception acknowledge signal in the order from the highest priority to the lowest one. The transmitting node collates the returned reception acknowledge signals with the transmission function address-physical address correlation table to discriminate if the reception acknowledge signals have properly been returned from the destination nodes specified by the function address.
According to the second prior art, the conventional problem inherent to the first prior art can be solved, but each node should store the transmission function address-physical address correlation table and the reception function address table, thus requiring a large capacity memory and a memory control circuit. This increases the manufacturing cost of the transmission system.